White-light spectral biometric sensors

ABSTRACT

Methods and systems are provided for performing a biometric function. A purported skin site of an individual is illuminated with white light. Light scattered from the purported skin site is received with a color imager on which the received light is incident. Spatially distributed images of the purported skin site are derived and correspond to different volumes of illuminated tissue of the individual. The images are analyzed to perform the biometric function.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 11/458,607, entitled “WHITE-LIGHT SPECTRAL BIOMETRIC SENSORS,”filed Jul. 19, 2006, by Robert K. Row et al. (“the 607 application”) Theentire disclosure of which is incorporated herein by reference for allpurposes.

The '607 application is a continuation-in-part of copending U.S. patentapplication No. 10/818,698, entitled “MULTISPECTRAL BIOMETRIC SENSOR,”filed Apr. 5, 2004 by Robert K. Rowe et al. (“the '698 application”).The '698 application is a nonprovisional of each of the followingprovisional applications: U.S. Prov. Pat. Appl. No. 60/460,247, entitled“NONINVASIVE ALCOHOL MONITOR,” filed Apr. 4, 2003; U.S. Prov. Pat. Appl.No. 60/483,281, entitled “HYPERSPECTRAL FINGERPRINT READER,” filed Jun.27, 2003 by Robert K. Rowe et al.; U.S. Prov. Pat. Appl. No. 60/504,594,entitled “HYPERSPECTRAL FINGERPRINTING,” filed Sep. 18, 2003; and U.S.Prov. Pat. Appl. No. 60/552,662, entitled “OPTICAL SKIN SENSOR FORBIOMETRICS,” filed Mar. 10, 2004. The entire disclosure of each of theforegoing applications is incorporated herein by reference for allpurposes.

The '607 application is also a continuation-in-part of copending U.S.patent application Ser. No. 11/219,006, entitled “COMPARATIVE TEXTUREANALYSIS OF TISSUE FOR BIOMETRIC SPOOF DETECTION,” filed Sep. 1, 2005 byRobert K. Rowe (“the '006 application”). The '006 application is acontinuation-in-part of U.S. patent application Ser. No. 10/818,698,entitled “MULTISPECTRAL BIOMETRIC SENSOR,” filed Apr. 5, 2004 by RobertK. Row. et al., which is a nonprovisional of each of U.S. Prov. Pat.Appl. No. 60/460,247, filed Apr. 4, 2003, U.S. Prov. Pat. Appl. No.60/483,281, filed Jun. 27, 2003, U.S. Prov. Pat. Appl. No. 60/504,594,filed Sep. 18, 2003, and U.S. Prov. Pat. Appl. No. 60/552,662, filedMar. 10, 2004. The entire disclosure of each of the foregoingapplications is incorporated herein by reference for all purposes.

The '006 application is also a continuation-in-part of U.S. patentapplication Ser. No. 11/115,100, entitled “MULTISPECTRAL IMAGINGBIOMETRICS,” filed Apr. 25, 2005 by Robert K. Rowe, which is anonprovisional of each of U.S. Prov. Pat. Appl. No. 60/576,364, filedJun. 1, 2004, U.S. Prov. Pat. Appl. No. 60/600,867, filed Aug. 11, 2004,U.S. Prov. Pat. Appl. No. 60/610,802, filed Sep. 17, 2004, U.S. Prov.Pat. Appl. No. 60/654,354, filed Feb. 18, 2005, and U.S. Prov. Pat.Appl. No. 60/659,024, filed Mar. 4, 2005. The entire disclosure of eachof the foregoing applications is incorporated herein by reference forall purposes.

The '006 application is also a continuation-in-part of U.S. patentapplication Ser. No. 11/115,101, entitled “MULTISPECTRAL BIOMETRICIMAGING,” filed Apr. 25, 2005 by Robert K. Rowe and Stephen P. Corcoran,which is a nonprovisional of each of U.S. Prov. Pat. Appl. No.60/576,364, filed Jun. 1, 2004, U.S. Prov. Pat. Appl. No. 60/600,867,filed Aug. 11, 2004, U.S. Prov. Pat. Appl. No. 60/610,802, filed Sep.17, 2004, U.S. Prov. Pat. Appl. No. 60/654,354, filed Feb. 18, 2005, andU.S. Prov. Pat. Appl. No. 60/659,024, filed Mar. 4, 2005. The entiredisclosure of each of the foregoing references is incorporated herein byreference for all purposes.

The '006 application is also a continuation-in-part of U.S. patentapplication Ser. No. 11/115,075, entitled “MULTISPECTRAL LIVENESSDETERMINATION,” FILED Apr. 25, 2005 by Robert K. Rowe, which is anonprovisional of each of U.S. Prov. Pat. Appl. No. 60/576,364, filedJun. 1, 2004, U.S. Prov. Pat. Appl. No. 60/600,867, filed Aug. 11, 2004,U.S. Prov. Pat. Appl. No. 60/610,802, filed Sep. 17, 2004, U.S. Prov.Pat. Appl. No. 60/654,354, filed Feb. 18, 2005, and U.S. Prov. Pat.Appl. No. 60/659,024, filed Mar. 4, 2005. The entire disclosure of eachof the foregoing references is incorporated herein by reference for allpurposes.

This application is also related to each of the following applications,the entire disclosure of each of which is incorporated herein byreference for all purposes: concurrently filed U.S. patent applicationSer. No. 11/458,619, entitled “TEXTURE-BIOMETRICS SENSOR,” filed byRobert K. Rowe; and U.S. patent application Ser. No. 09/874,740,entitled “APPARATUS AND METHOD OF BIOMETRIC DETERMINATION USINGSPECIALIZED OPTICAL SPECTROSCOPY SYSTEM,” filed Jun. 5, 2001.

BACKGROUND OF THE INVENTION

This application relates generally to biometrics. More specifically,this application relates to methods and systems for performing biometricmeasurements that use spectral information.

“Biometrics” refers generally to the statistical analysis ofcharacteristics of living bodies. One category of biometrics includes“biometric identification,” which commonly operates under one of twomodes to provide automatic identification of people or to verifypurported identities of people. Biometric sensing technologies measurethe physical features or behavioral characteristics of a person andcompare those features to similar prerecorded measurements to determinewhether there is a match. Physical features that are commonly used forbiometric identification include faces, irises, hand geometry, veinstructure, and fingerprint patterns, which is the most prevalent of allbiometric-identification features. Current methods for analyzingcollected fingerprints include optical, capacitive, radio-frequency,thermal, ultrasonic, and several other less common techniques.

Most of the fingerprint-collection methods rely on measuringcharacteristics of the skin at or very near the surface of a finger. Inparticular, optical fingerprint readers typically rely on the presenceor absence of a difference in the index of refraction between the sensorplaten and the finger placed on it. When an air-filled valley of thefingerprint is above a particular location of the platen, total internalreflectance (“TIR”) occurs in the platen because of the air-platen indexdifference. Alternatively, if skin of the proper index of refraction isin optical contact with the platen, then the TIR at this location is“frustrated,” allowing light to traverse the platen-skin interface. Amap of the differences in TIR across the region where the finger istouching the platen forms the basis for a conventional opticalfingerprint reading. There are a number of optical arrangements used todetect this variation of the optical interface in both bright-field anddark-field optical arrangements. Commonly, a single, quasimonochromaticbeam of light is used to perform this TIR-based measurement.

There also exists non-TIR optical fingerprint sensors. In most cases,these sensors rely on some arrangement of quasimonochromatic light toilluminate the front, sides, or back of a fingertip, causing the lightto diffuse through the skin. The fingerprint image is formed due to thedifferences in light transmission across the skin-platen boundary forthe ridge and valleys. The difference in optical transmission are due tochanges in the Fresnel reflection characteristics due to the presence orabsence of any intermediate air gap in the valleys, as known to one offamiliarity in the art.

Optical fingerprint readers are particularly susceptible to imagequality problems due to non-ideal conditions. If the skin is overly dry,the index match with the platen will be compromised, resulting in poorimage contrast. Similarly, if the finger is very wet, the valleys mayfill with water, causing an optical coupling to occur all across thefingerprint region and greatly reducing image contrast. Similar effectsmay occur if the pressure of the finger on the platen is too little ortoo great, the skin or sensor is dirty, the skin is aged and/or worn, oroverly fine features are present such as may be the case for certainethnic groups and in very young children. These effects decrease imagequality and thereby decrease the overall performance of the fingerprintsensor. In some cases, commercial optical fingerprint readersincorporate a thin membrane of soft material such as silicone to helpmitigate these effects and restore performance. As a soft material, themembrane is subject to damage, wear, and contamination, limiting the useof the sensor without maintenance.

Optical fingerprint readers, such as those based on TIR, as well asother modalities such as capacitance, RF, and others, typically produceimages that are affected to some degree by the nonideal imagingconditions present during acquisition. An analysis of the texturalcharacteristics of the resulting images is thus affected by the samplingconditions, which may limit or obscure the ability to observe thetextural characteristics of the person's skin. The consequence of thisis that texture is of limited utility in such sensing modalities.

Biometric sensors, particularly fingerprint biometric sensors, aregenerally prone to being defeated by various forms of spoof samples. Inthe case of fingerprint readers, a variety of methods are known in theart for presenting readers with a fingerprint pattern of an authorizeduser that is embedded in some kind of inanimate material such as paper,gelatin, epoxy, latex, and the like. Thus, even if a fingerprint readercan be considered to reliably determine the presence or absence of amatching fingerprint pattern, it is also critical to the overall systemsecurity to ensure that the matching pattern is being acquired from agenuine, living finger, which may be difficult to ascertain with manycommon sensors.

Another way in which some biometric systems may be defeated is throughthe use of a replay attack. In this scenario, an intruder records thesignals coming from the sensor when an authorized user is using thesystem. At a later time, the intruder manipulates the sensor system suchthat the prerecorded authorized signals may be injected into the system,thereby bypassing the sensor itself and gaining access to the systemsecured by the biometric.

A common approach to making biometric sensors more robust, more secure,and less error-prone is to combine sources of biometric signals using anapproach sometimes referred to in the art as using “dual,”“combinatoric,” “layered,” “fused,” “multibiometric,” or “multifactorbiometric” sensing. To provide enhanced security in this way, biometrictechnologies are combined in such a way that different technologiesmeasure portions of the body at the same time and are resistant to beingdefeated by using different samples or techniques to defeat thedifferent sensors that are combined. When technologies are combined in away that they view the same part of the body they are referred to asbeing “tightly coupled.”

The accuracy of noninvasive optical measurements of physiologicalanalytes such as glucose, alcohol, hemoglobin, urea, and cholesterol canbe adversely affected by variation of the skin tissue. In some cases itis advantageous to measure one or more physiological analytes inconjunction with a biometric measurement. Such dual measurement haspotential interest and application to both commercial andlaw-enforcement markets.

There is accordingly a general need in the art for improved methods andsystems for biometric sensing and analyte estimation using multispectralimaging systems and methods.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention provide methods and systems for performingbiometric functions. White light is used to illuminate a purported skinsite and a color imager is used to collect light scattered from thepurported skin site for the generation of multispectral data. Thesemultispectral data may be generated in the form of multiple images ofthe skin site collected with different illumination wavelengths, whichcorrespond to different volumes of illuminated tissue. These data arethen subjected to different types of analyses depending on specificaspects of the biometric function to be performed.

Thus, in a first set of embodiments, a biometric sensor is provided. Awhite-light illumination subsystem is disposed to illuminate a purportedskin site of an individual with white light. A detection subsystem isdisposed to receive light scattered from the purported skin site andcomprises a color imager on which the received light is incident. Acomputational unit is interfaced with the detection subsystem. Thecomputational unit has instructions for deriving a plurality ofspatially distributed images of the purported skin site from thereceived light with the color imager. The plurality of spatiallydistributed images correspond to different volumes of illuminated tissueof the individual. The computational unit also has instructions foranalyzing the plurality of spatially distributed images to perform abiometric function.

In one of these embodiments, the biometric function comprises anantispoofing function and the instructions for analyzing the pluralityof spatially distributed images comprise instructions for determiningwhether the purported skin site comprises living tissue. In another ofthese embodiments, the instructions for analyzing the plurality ofspatially distributed images to perform the biometric function compriseinstructions for analyzing the plurality of spatially distributed imagesto estimate a demographic or anthropometric characteristic of theindividual. In still another of these embodiments, the instructions foranalyzing the plurality of spatially distributed images to perform thebiometric function comprise instructions for analyzing the plurality ofspatially distributed images to determine a concentration of an analytein blood of the individual.

In some embodiments, the biometric sensor may further comprise a platenin contact with the purported skin site, with the white-lightillumination subsystem being adapted to illuminate the purported skinsite through the platen. In other embodiments, the white-lightillumination subsystem may instead be adapted to illuminate thepurported skin site when the skin site is not in physical contact withthe biometric sensor.

The white light may be provided in different ways in differentembodiments. For example, in one embodiment, the white-lightillumination subsystem comprises a broadband source of white light. Inanother embodiment, the white-light illumination subsystem comprises aplurality of narrow-band light sources and an optical arrangement tocombine light provided by the plurality of narrow-band light sources.The plurality of narrow-band light sources may provide light atwavelengths that correspond to each of a set of primary colors. In somecases, the purported skin site and an illumination region where thepurported skin site is illuminated are in relative motion.

Some embodiments make use of polarization by including a firstpolarizing in the illumination system disposed to polarize the whitelight. The detection system then comprises a second polarizer disposedto encounter the received light. The first and second polarizers may becrossed relative to each other. In other embodiments, the first andsecond polarizers may be parallel. In some embodiments, the firstpolarizer may be omitted while retaining the second. In someembodiments, two or more of these polarization options may be combinedin a single device. The detection system may also sometimes include aninfrared filter disposed to encounter the received light before thereceived light is incident on the color imager.

In certain instances, the purported skin site is a volar surface of afinger or hand and the biometric function comprises a biometricidentification. The instructions for analyzing the plurality ofspatially distributed images comprise instructions for deriving asurface fingerprint or palmprint image of the purported skin site fromthe plurality of spatially distributed images. The surface fingerprintor palmprint image is then compared with a database of fingerprint orpalmprint images to identify the individual. In other embodiment wherethe biometric function comprises a biometric identification, theinstructions for analyzing the plurality of spatially distributed imagesinstead comprise instructions for comparing the plurality of spatiallydistributed images with a database of multispectral images to identifythe individual.

In a second set of embodiments, a method is provided of performing abiometric function. A purported skin site of an individual isilluminated with white light. Light scattered from the purported skinsite is received with a color imager on which the received light isincident. A plurality of spatially distributed images of the purportedskin site are derived, with the plurality of spatially distributedimages corresponding to different volumes of illuminated tissue of theindividual. The plurality of spatially distributed images are analyzedto perform the biometric function.

In some of these embodiments, the biometric function comprises anantispoofing function and analyzing the plurality of spatiallydistributed images comprises determining whether the purported skin sitecomprises living tissue. In other of these embodiments, the plurality ofspatially distributed images are analyzed to estimate a demographic oranthropometric characteristic of the individual. In still different onesof these embodiments, the plurality of spatially distributed images areanalyzed to determine a concentration of an analyte in blood of theindividual.

The purported skin site may sometimes be illuminated by directing thewhite light through a platen in contact with the purported skin site. Insome instances, the purported skin site may be illuminated with abroadband source of white light, while in other instances a plurality ofnarrow-band beams, perhaps corresponding to a set of primary colors, maybe generated and combined. The purported skin site might sometimes be inrelative motion with an illumination region where the purported skinsite is illuminated.

In one embodiment the while light is polarized with a first polarizationand the received light scattered from the purported skin site ispolarized with a second polarization. The first and second polarizationsmay be substantially crossed relative to each other or may besubstantially parallel to each other. The received light may sometimesbe filtered at infrared wavelengths before the received light isincident on the color imager.

In some instances, the biometric function comprises a biometricidentification. For instance, the purported skin site could be a volarsurface of a finger or hand. Analysis of the plurality of spatiallydistributed images could then proceed by deriving a surface fingerprintor palmprint image of the purported skin site from the plurality ofspatially distributed images and comparing the surface fingerprint orpalmprint image with a database of fingerprint or palmprint images. Inan alternative embodiment, the plurality of spatially distributed imagescould be compared with a database of multispectral images to identifythe individual.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings wherein like reference labels are usedthroughout the several drawings to refer to similar components. In someinstances, reference labels include a numerical portion followed by alatin-letter suffix; reference to only the numerical portion ofreference labels is intended to refer collectively to all referencelabels that have that numerical portion but different latin-lettersuffices.

FIG. 1 provides a front view of a noncontact biometric sensor in oneembodiment of the invention;

FIG. 2A provides an illustration of a structure for a Bayer color filterarray, which may be used in embodiments of the invention;

FIG. 2B is a graph showing color response curves for a Bayer colorfilter array like that illustrated in FIG. 2A;

FIG. 3 provides a front view of a noncontact biometric sensor in anotherembodiment of the invention;

FIG. 4 provides a top view of a sensor configuration that collects dataduring relative motion between a skin site and an optically activeregion of the sensor;

FIG. 5 illustrates a multispectral datacube that may be used in certainembodiments of the invention;

FIG. 6 is a front view of a contact biometric sensor in one embodimentof the invention;

FIG. 7A provides a side view of a contact biometric sensor in anembodiment;

FIG. 7B provides a side view of a contact biometric sensor in anotherembodiment;

FIG. 8 provides a front view of a contact biometric sensor in a furtherembodiment of the invention;

FIG. 9A illustrates a structure for a contact texture biometric sensorin an embodiment of the invention;

FIG. 9B provides a side view of a contact texture biometric sensor inone configuration;

FIG. 9C provides a side view of a contact texture biometric sensor inanother configuration;

FIG. 10 is schematic representation of a computer system that may beused to manage functionality of contact and noncontact biometric sensorsin accordance with embodiments of the invention;

FIG. 11 is a flow diagram summarizing methods of using contact andnoncontact biometric sensors and illustrates a number of differentbiometric functions that may be performed; and

FIG. 12 is a flow diagram summarizing methods of operation of contacttexture biometric sensors in accordance with embodiments of theinvention.

DETAILED DESCRIPTION OF THE INVENTION 1. Overview

Embodiments of the invention provide methods and systems that allow forthe collection and processing of a variety of different types ofbiometric measurements, including integrated, multifactor biometricmeasurements in some embodiments. These measurements may provide strongassurance of a person's identity, as well as of the authenticity of thebiometric sample being taken. In some embodiments, a sensor uses whitelight that penetrates the surface of the person's skin, and scatterswithin the skin and/or the underlying tissue. As used herein, “whitelight” refers to light that has a spectral composition amenable toseparation into constituent wavelength bands, which in some cases maycomprise primary colors. The usual primary colors used to define whitelight are red, green, and blue, but other combinations may be used inother instances, as will be known to those of skill in the art. Forclarity, it is emphasized that “white light” as used herein might notappear white to a human observer and might have a distinct tint or colorassociated with it because of the exact wavelength distribution andintensity of the constituent wavelength bands. In other cases, the whitelight may comprise one or more bands in the ultraviolet or infraredspectral regions. In some cases, the white light might not even bevisible at all to a human observer when it consists of wavelength bandsin the infrared and/or ultraviolet spectral regions. A portion of thelight scattered by the skin and/or underlying tissue exits the skin andis used to form an image of the structure of the tissue at and below thesurface of the skin. Because of the wavelength-dependent properties ofthe skin, the image formed from each wavelength of light comprised bythe white light may be different from images formed at otherwavelengths. Accordingly, embodiments of the invention collect images insuch a way that characteristic spectral and spatial information may beextracted from the resulting image.

In some applications, it may be desirable to estimate other parametersand characteristics of a body, either independently or in combinationwith a biometric measurement. For example, in one specific suchembodiment, an ability is provided to measure analyte levels of a personsimultaneously with measurement of a fingerprint pattern. Applicationsto law enforcement may be found in embodiments where the measure analytecomprises a blood-alcohol level of the person; such embodiments alsoenable a variety of commercial applications that include restrictingmotor-vehicle access. In this way, the analyte measurement and theidentity of the person on whom the measurement is made may beinextricably linked.

Skin composition and structure is very distinct, very complex, andvaries from person to person. By performing optical measurements of thespatiospectral properties of skin and underlying tissue, a number ofassessments may be made. For example, a biometric-identificationfunction may be performed to identify or verify whose skin is beingmeasured, a liveness function may be performed to assure that the samplebeing measured is live and viable skin and not another type of material,estimates may be made of a variety of physiological parameters such asage gender, ethnicity, and other demographic and anthropometriccharacteristics, and/or measurements may be made of the concentrationsof various analytes and parameters including alcohol, glucose, degreesof blood perfusion and oxygenation, biliruben, cholesterol, urea, andthe like.

The complex structure of skin may be used in different embodiments totailor aspects of the methods and systems for particular functions. Theoutermost layer of skin, the epidermis, is supported by the underlyingdermis and hypodermis. The epidermis itself may have five identifiedsublayers that include the stratum corneum, the stratum lucidum, thestratum granulosum, the stratum spinosum, and the stratum germinativum.Thus, for example, the skin below the top-most stratum corneum has somecharacteristics that relate to the surface topography, as well as somecharacteristics that change with depth into the skin. While the bloodsupply to skin exists in the dermal layer, the dermis has protrusionsinto the epidermis known as “dermal papillae,” which bring the bloodsupply close to the surface via capillaries. In the volar surfaces ofthe fingers, this capillary structure follows the pattern of thefriction ridges and valleys on the surface. In some other locations onthe body, the structure of the capillary bed may be less ordered, but isstill characteristic of the particular location and person. As well, thetopography of the interface between the different layers of skin isquite complex and characteristic of the skin location and the person.While these sources of subsurface structure of skin and underlyingtissue represent a significant noise source for non-imaging opticalmeasurements of skin for biometric determinations or analytemeasurements, the structural differences are manifested byspatiospectral features that can be compared favorably throughembodiments of the invention.

In some instances, inks, dyes and/or other pigmentation may be presentin portions of the skin as topical coating or subsurface tattoos. Theseforms of artificial pigmentation may or may not be visible to the nakedhuman eye. However, if one or more wavelengths used by the apparatus ofthe present invention is sensitive to the pigment, the sensor can beused in some embodiments to verify the presence, quantity and/or shapeof the pigment in addition to other desired measurement tasks.

In general, embodiments of the present invention provide methods andsystems that collect spatiospectral information that may be representedin a multidimensional data structure that has independent spatial andspectral dimensions. In certain instances, the desired information iscontained in just a portion of the entire multidimensional datastructure. For example, estimation of a uniformly distributed,spectrally active compound may require just the measured spectralcharacteristics, which may be extracted from the overallmultidimensional data structure. In such cases, the overall systemdesign may be simplified to reduce or eliminate the spatial component ofthe collected data by reducing the number of image pixels, even to alimit of a single pixel. Thus, while the systems and methods disclosedare generally described in the context of spatiospectral imaging, itwill be recognized that the invention encompasses similar measurementsin which the degree of imaging is greatly reduced, even to the pointwhere there is a single detector element.

2. Noncontact Biometric Sensors

One embodiment of the invention is depicted with the schematic diagramof FIG. 1, which shows a front view of a noncontact biometric sensor101. The sensor 101 comprises an illumination subsystem 121 having oneor more light sources 103 and a detection subsystem 123 with an imager115. The figure depicts an embodiment in which the illuminationsubsystem 121 comprises a plurality of illumination subsystems 121 a and121 b, but the invention is not limited by the number of illumination ordetection subsystems 121 or 123. For example, the number of illuminationsubsystems 121 may conveniently be selected to achieve certain levels ofillumination, to meet packaging requirements, and to meet otherstructural constraints of the sensor 101. Illumination light passes fromthe source 103 through illumination optics 105 that shape theillumination to a desired form, such as in the form of flood light,light lines, light points, and the like. The illumination optics 105 areshown for convenience as consisting of a lens but may more generallyinclude any combination of one or more lenses, one or more mirrors,and/or other optical elements. The illumination optics 105 may alsocomprise a scanner mechanism (not shown) to scan the illumination lightin a specified one-dimensional or two-dimensional pattern. The lightsource 103 may comprise a point source, a line source, an area source,or may comprise a series of such sources in different embodiments. Inone embodiment, the illumination light is provided as polarized light,such as by disposing a linear polarizer 107 through which the lightpasses before striking a finger 119 or other skin site of the personbeing studied. Embodiments like those shown in FIG. 1 are referred toherein as “noncontact” sensors because the imaged skin site may bepositioned to interact with the light without being in contact with anysolid surface. In “contact” biometric sensors described in detail below,the imaged skin site is in contact with some solid surface such as aplaten or light detector.

In some instances, the light source 103 comprises a white-light source,which may be provided as a broad-band source or as a collection ofnarrow-band emitters in different embodiments. Examples of broad-bandsources include white-light emitting diodes (“LEDs”), incandescent bulbsor glowbars, and the like. Collections of narrow-band emitters maycomprise quasimonochromatic light sources having primary-colorwavelengths, such as in an embodiment that includes a red LED or laserdiode, a green LED or laser diode, and a blue LED or laser diode.

An alternative mechanism for reducing the directly reflected light makesuse of optical polarizers. Both linear and circular polarizers can beemployed advantageously to make the optical measurement more sensitiveto certain skin depths, as known to on familiar in the art. In theembodiment illustrated in FIG. 1, the illumination light is polarized bylinear polarizer 107. The detection subsystem 123 may then also includea linear polarizer 111 that is arranged with its optical axissubstantially orthogonal to the illumination polarizer 107. In this way,light from the sample must undergo multiple scattering events tosignificantly change it state of polarization. Such events occur whenthe light penetrates the surface of the skin and is scattered back tothe detection subsystem 123 after many scatter events.

Conversely, the use of two polarizers 107 and 111 may also be used toincrease the influence of directly reflected light by arranging thepolarizer 111 to be substantially parallel to polarizer 107. In somesystems, it may be advantageous to combine two or more polarizationconfigurations in a single device to enable the collection ofmultispectral data collected under two different polarization conditions(i.e. under crossed-polarization and under parallel-polarizationconditions). In other embodiments, either polarizer 107 or 111, or both,may be omitted, allowing for the collection of substantially randomlypolarized light.

The detection subsystem 123 may incorporate detection optics thatcomprise lenses, mirrors, phase plates and wavefront coding devices,and/or other optical elements that form an image onto the detector 115.The detection optics 113 may also comprise a scanning mechanism (notshown) to relay portions of the overall image onto the detector 115 insequence. In all cases, the detection subsystem 123 is configured to besensitive to light that has penetrated the surface of the skin andundergone optical scattering within the skin and/or underlying tissuebefore exiting the skin.

In embodiments where white light is used, the detector 115 may comprisea Bayer color filter array in which filter elements corresponding to aset of primary colors are arranged in a Bayer pattern. An example ofsuch a pattern is shown in FIG. 2A for an arrangement that uses red 204,green 212, and blue 208 color filter elements. In some instances, thedetector subsystem 123 may additionally comprise an infrared filter 114disposed to reduce the amount of infrared light detected. As seen fromthe color response curve for a typical Bayer filter array shown in FIG.2B, there is generally some overlap in the spectral ranges of the red224, green 232, and blue 228 transmission characteristics of the filterelements. As evident particularly in the curves for the green 232 andblue 228 transmission characteristics, the filter array may allow thetransmission of infrared light. This is avoided with the inclusion of aninfrared filter 114 as part of the detector subsystem. In otherembodiments, the infrared filter 114 may be omitted and one or morelight sources 103 that emit infrared light may be incorporated. In thisway, all color filter elements 204, 208, and 212 may allow the light tosubstantially pass through, resulting in an infrared image across theentire detector 115.

Another embodiment of a noncontact biometric sensor is shownschematically with the front view of FIG. 3. In this embodiment, thebiometric sensor 301 comprises an illumination subsystem 323 and adetection subsystem 325. Similar to the embodiment described inconnection with FIG. 1, there may be multiple illumination subsystems323 in some embodiments, with FIG. 3 showing a specific embodimenthaving two illumination subsystems 323. A white-light source 303comprised by the illumination subsystem 323 may be any source of whitelight, including the broad-band or combination of narrow-band sourcesdescribed above. Light from the white-light source 303 passes throughillumination optics 305 and a linear polarizer 307 before passing intothe skin site 119. A portion of the light is diffusely reflected fromthe skin site 119 into the detection subsystem 325, which comprisesimaging optics 315 and 319, a linear polarizer 311, and a dispersiveoptical element 313. The dispersive element 313 may comprise a one- ortwo-dimensional grating, which may be transmissive or reflective, aprism, or any other optical component known in the art to cause adeviation of the path of light as a function of the light's wavelength.In the illustrated embodiment, the first imaging optics 319 acts tocollimate light reflected from the skin site 119 for transmissionthrough the linear polarizer 311 and dispersive element 313. Spectralcomponents of the light are angularly separated by the dispersiveelement 313 and are separately focused by the second imaging optics 315onto a detector. As discussed in connection with FIG. 1, when theoptical axis of the polarizers 307 and 311 are oriented to besubstantially orthogonal to each other, polarizers 307 and 311respectively comprised by the illumination and detection subsystems 323and 325 act to reduce the detection of directly reflected light at thedetector 317. The polarizers 307, 311 may also be oriented such thattheir optical axes are substantially parallel, which will increase thedetection of directly reflected light at the detector 317. In someembodiments, either polarizer 307 or 311, or both, may be omitted.

The image generated from light received at the detector is thus a“coded” image in the manner of a computer tomographic imagingspectrometer (“CTIS”). Both spectral and spatial information aresimultaneously present in the resulting image. The individual spectralpatters may be obtained by mathematical inversion or “reconstruction” ofthe coded image.

The description of the contactless sensor of FIG. 1 noted that a scannermechanism may be provided to scan the illumination light. This is anexample of a more general class of embodiments in which there isrelative motion of the illumination region and skin site. In suchembodiments, the image may be constructed by building up separate imageportions collected during the relative motion. Such relative motion mayalso be achieved in embodiments that configure the sensor in a swipeconfiguration, in which the user is instructed to translate the skinsite. One example of a swipe sensor is shown in top view with theschematic illustration of FIG. 4. In this figure, the illuminationregion and detection region 405 of a sensor 401 are substantiallycollinear. In some embodiments of a swipe sensor 401, there may be morethan a single illumination region. For example, there may be a pluralityof illumination regions arranged on either side of the detection region405. In some embodiments, the illumination region 403 may partially orfully overlay the detection region. The image data are collected withthe sensor by translating a finger or other body part through theoptically active region, as indicated by the arrow in FIG. 4. A swipesensor may be implemented with any of the contactless sensorconfigurations described above, although in some implementations it maybe used with a contact configuration, examples of which are described indetail below. The light that is received sequentially from discreteportions of the skin site is used to build up the image that issubsequently used for biometric applications.

The embodiments described above produce a body of spatio-spectral data,which may be used in biometrics applications as described below. Theinvention is not limited to any particular manner of storing oranalyzing the body of spatio-spectral data. For purposes ofillustration, it is shown in the form of a datacube in FIG. 5. Thedatacube 501 is shown decomposed along a spectral dimension with aplurality of planes 503, 505, 507, 509, 511, each of which correspondsto a different portion of the light spectrum and each of which includespatial information. In some instances, the body of spatio-spectral datamay include additional types of information beyond spatial and spectralinformation. For instance, different illumination conditions as definedby different illumination structures, different polarizations, and thelike may provide additional dimensions of information. More broadly,data collected under a plurality of optical conditions, whether they becollected simultaneously or sequentially, is referred to herein as“multispectral” data. A more complete description of aspects ofmultispectral data is described in copending, commonly assigned U.S.patent application Ser. No. 11/379,945, entitled “MULTISPECTRALBIOMETRIC SENSORS,” filed Apr. 24, 2006, the entire disclosure of whichis incorporated herein by reference for all purposes. Spatio-spectraldata may thus be considered to be a subset of certain types ofmultispectral data where the different optical conditions includedifferent illumination wavelengths.

In an embodiment where illumination takes place under white light, theimages 503, 505, 507, 509, and 511 might correspond, for example toimages generated using light at 450 nm, 500 nm, 550 nm, 600 nm, and 650nm. In another example, there may be three images that correspond to theamount of light in the red, green, and blue spectral bands at each pixellocation. Each image represents the optical effects of light of aparticular wavelength interacting with skin. Due to the opticalproperties of skin and skin components that vary by wavelength, each ofthe multispectral images 503, 505, 507, 509, and 511 will be, ingeneral, different from the others. The datacube may thus be expressedas R(X_(S), Y_(S), X_(I), Y_(I), λ) and describes the amount ofdiffusely reflected light of wavelength λ seen at each image pointX_(I), Y_(I) when illuminated at a source point X_(S), Y_(S). Differentillumination configurations (flood, line, etc.) can be summarized bysumming the point response over appropriate source point locations. Aconventional non-TIR fingerprint image F(X_(I), Y_(I)) can loosely bedescribed as the multispectral data cube for a given wavelength, λ_(o),and summed over all source positions:

${F( {X_{I},Y_{I}} )} = {\sum\limits_{Y_{S}}{\sum\limits_{X_{S}}{{R( {X_{S},Y_{S},X_{I},Y_{I},\lambda_{0}} )}.}}}$

Conversely, the spectral biometric dataset S(λ) relates the measuredlight intensity for a given wavelength λ to the difference {right arrowover (D)} between the illumination and detection locations:

S({right arrow over (D)}, λ)=R(X _(I) −X _(S) , Y _(I) −Y _(S), λ).

The datacube R is thus related to both conventional fingerprint imagesand to spectral biometric datasets. The datacube R is a superset ofeither of the other two data sets and contains correlations and otherinformation that may be lost in either of the two separate modalities.

The light that passes into the skin and/or underlying tissue isgenerally affected by different optical properties of the skin and/orunderlying tissue at different wavelengths. Two optical effects in theskin and/or underlying tissue that are affected differently at differentwavelengths are scatter and absorbance. Optical scatter in skin tissueis generally a smooth and relatively slowly varying function wavelength.Conversely, absorbance in skin is generally a strong function ofwavelength due to particular absorbance features of certain componentspresent in the skin. For example blood, melanin, water, carotene,biliruben, ethanol, and glucose all have significant absorbanceproperties in the spectral region from 400 nm to 2.5 μm, which maysometimes be encompassed by the white-light sources.

The combined effect of optical absorbance and scatter causes differentillumination wavelengths to penetrate the skin to different depths. Thiseffectively causes the different spectral images to have different andcomplementary information corresponding to different volumes ofilluminated tissue. In particular, the capillary layers close to thesurface of the skin have distinct spatial characteristics that can beimaged at wavelengths where blood is strongly absorbing. Because of thecomplex wavelength-dependent properties of skin and underlying tissue,the set of spectral values corresponding to a given image location hasspectral characteristics that are well-defined and distinct. Thesespectral characteristics may be used to classify the collected image ona pixel-by-pixel basis. This assessment may be performed by generatingtypical tissue spectral qualities from a set of qualified images. Forexample, the spatio-spectral data shown in FIG. 5 may be reordered as anN×5 matrix, where

N is the number of image pixels that contain data from living tissue,rather than from a surrounding region of air. An eigenanalysis or otherfactor analysis performed on this set matrix produces the representativespectral features of these tissue pixels. The spectra of pixels in alater data set may then be compared to such previously establishedspectral features using metrics such as Mahalanobis distance andspectral residuals. If more than a small number of image pixels havespectral qualities that are inconsistent with living tissue, then thesample is deemed to be non-genuine and rejected, thus providing amechanism for incorporating antispoofing methods in the sensor based ondeterminations of the liveness of the sample.

Alternatively, textural characteristics of the skin may alone or inconjunction with the spectral characteristics be used to determine theauthenticity of the sample. For example, each spectral image may beanalyzed in such a way that the magnitude of various spatialcharacteristics may be described. Methods for doing so include wavelettransforms, Fourier transforms, cosine transforms, gray-levelco-occurrence, and the like. The resulting coefficients from any suchtransform described an aspect of the texture of the image from whichthey were derived. The set of such coefficients derived from a set ofspectral images thus results in a description of the chromatic texturalcharacteristics of the multispectral data. These characteristics maythen be compared to similar characteristics of known samples to performa biometric determination such as spoof or liveness determination.Methods for performing such determinations are generally similar to themethods described for the spectral characteristics above. Applicableclassification techniques for such determinations include linear andquadratic discriminant analysis, classification trees, neural networks,and other methods known to those familiar in the art.

Similarly, in an embodiment where the sample is a volar surface of ahand or finger, the image pixels may be classified as “ridge,” “valley,”or “other” based on their spectral qualities or their chromatic texturalqualities. This classification can be performed using discriminantanalysis methods such as linear discriminant analysis, quadraticdiscriminant analysis, principal component analysis, neural networks,and others known to those of skill in the art. Since ridge and valleypixels are contiguous on a typical volar surface, in some instances,data from the local neighborhood around the image pixel of interest areused to classify the image pixel. In this way, a conventionalfingerprint image may be extracted for further processing and biometricassessment. The “other” category may indicate image pixels that havespectral qualities that are different than anticipated in a genuinesample. A threshold on the total number of pixels in an image classifiedas “other” may be set. If this threshold is exceeded, the sample may bedetermined to be non-genuine and appropriate indications made andactions taken.

In a similar way, multispectral data collected from regions such as thevolar surface of fingers may be analyzed to directly estimate thelocations of “minutiae points,” which are defined as the locations atwhich ridges end, bifurcate, or undergo other such topographic change.For example, the chromatic textural qualities of the multispectraldataset may be determined in the manner described above. These qualitiesmay then be used to classify each image location as “ridge ending,”“ridge bifurcation,” or “other” in the manner described previously. Inthis way, minutiae feature extraction may be accomplished directly fromthe multispectral data without having to perform computationallylaborious calculations such as image normalization, image binarization,image thinning, and minutiae filtering, techniques that are known tothose familiar in the art.

Biometric determinations of identity may be made using the entire bodyof spatio-spectral data or using particular portions thereof. Forexample, appropriate spatial filters may be applied to separate out thelower spatial frequency information that is typically representative ofdeeper spectrally active structures in the tissue. The fingerprint datamay be extracted using similar spatial frequency separation and/or thepixel-classification methods disclosed above. The spectral informationcan be separated from the active portion of the image in the mannerdiscussed above. These three portions of the body of spatio-spectraldata may then be processed and compared to the corresponding enrollmentdata using methods known to one familiar in the art to determine thedegree of match. Based upon the strength of match of thesecharacteristics, a decision can be made regarding the match of thesample with the enrolled data. Additional details regarding certaintypes of spatio-spectral analyses that may be performed are provided inU.S. patent application Ser. No. 10/818,698, entitled “MULTISPECTRALBIOMETRIC SENSOR,” filed Apr. 5, 2004 by Robert K. Rowe et al., theentire disclosure of which is incorporated herein by reference for allpurposes.

As previously noted, certain substances that may be present in the skinand underlying tissue have distinct absorbance characteristics. Forexample, ethanol has characteristic absorbance peaks at approximately2.26 μm, 2.30 μm, and 2.35 μm, and spectral troughs at 2.23 μm, 2.28 μm,2.32 μm, and 2.38 μm. In some embodiments, noninvasive opticalmeasurements are performed at wavelengths in the range of 2.1-2.5 μm,more particularly in the range of 2.2-2.4 μm. In an embodiment thatincludes at least one of the peak wavelengths and one of the troughwavelengths, the resulting spectral data are analyzed using multivariatetechniques such as partial least squares, principal-componentregression, and others known to those of skill in the art, to provide anestimate of the concentration of alcohol in the tissue, as well as toprovide a biometric signature of the person being tested. While acorrelation to blood-alcohol level may be made with values determinedfor a subset of these wavelengths, it is preferable to test at least thethree spectral peak values, with more accurate results being obtainedwhen the seven spectral peak and trough values are measured.

In other embodiments, noninvasive optical measurements are performed atwavelengths in the range of 1.5-1.9 μm, more particularly in the rangeof 1.6-1.8 μm. In specific embodiments, optical measurements areperformed at one or more wavelengths of approximately 1.67 μm, 1.69 μm,1.71 μm, 1.73 μm, 1.74 μm 1.76 μm and 1.78 μm. The presence of alcoholis characterized at these wavelengths by spectral peaks at 1.69 μm, 1.73μm, and 1.76 μm and by spectral troughs at 1.67 μm, 1.71 μm, 1.74 μm,and 1.78 μm. Similar to the 2.1-2.5 μm wavelength range, theconcentration of alcohol is characterized by relative strengths of oneor more of the spectral peak and trough values. Also, while acorrelation to blood-alcohol level may be made with values determinedfor a subset of these wavelengths in the 1.5-1.9 μm range, it ispreferable to test at least the three spectral peak values, with moreaccurate results being obtained when the seven spectral peak and troughvalues are measured.

A small spectral alcohol-monitoring device may be embedded in a varietyof systems and applications in certain embodiments. The spectralalcohol-monitoring device can be configured as a dedicated system suchas may be provided to law-enforcement personnel, or may be integrated aspart of an electronic device such as an electronic fob, wristwatch,cellular telephone, PDA, or any other electronic device, for anindividual's personal use. Such devices may include mechanisms forindicating to an individual whether his blood-alcohol level is withindefined limits. For instance, the device may include red and green LEDs,with electronics in the device illuminating the green LED if theindividual's blood-alcohol level is within defined limits andilluminating the red LED if it is not. In one embodiment, thealcohol-monitoring device may be included in a motor vehicle, typicallypositioned so that an individual may conveniently place tissue, such asa fingertip, on the device. While in some instances, the device mayfunction only as an informational guide indicating acceptability todrive, in other instances ignition of the motor vehicle mayaffirmatively depend on there being a determination that the individualhas a blood-alcohol level less than a prescribed level.

3. Contact Biometric Sensors

Biometric sensors may be constructed in a fashion similar to that shownin FIGS. 1 and 3, but configured so that the skin site is placed incontact with a platen. Such designs have certain additionalcharacteristics that result from the interaction of light with theplaten, sometimes permitting additional information to be incorporatedas part of the collect spatio-spectral data.

One embodiment is shown in FIG. 6, which provides a front view of acontact biometric sensor 601. Like the sensor illustrated in FIG. 1, thecontact sensor 601 has one or more illumination subsystems 621 and adetection subsystem 623. Each of the illumination subsystems 621comprises one or more white-light sources 603 and illumination opticsthat shape the light provided by the sources 603 into a desired form. Aswith the non-contact arrangements, the illumination optics may generallyinclude any combination of optical elements and may sometimes include ascanner mechanism. In some instances, the illumination light is providedas polarized light by disposing a polarizer 607 through which theillumination light passes. Examples of white-light sources 603,including broad- and narrow-band sources were described above, and thesources 603 may be configured to provide sources having different shapesin different embodiments.

The illumination light is directed by the illumination optics 621 topass through a platen 617 and illuminate the skin site 119. The sensorlayout 601 and components may advantageously be selected to minimize thedirect reflection of the illumination optics 621. In one embodiment,such direct reflections are reduced by relatively orienting theillumination subsystem 621 and detection subsystem 623 such that theamount of directly reflected light detected is minimized. For instance,optical axes of the illumination subsystem 621 and the detectionsubsystem 623 may be placed at angles such that a mirror placed on theplaten 617 does not direct an appreciable amount of illumination lightinto the detection subsystem 623. In addition, the optical axes of theillumination and detection subsystems 621 and 623 may be placed atangles relative to the platen 617 such that the angular acceptance ofboth subsystems is less than the critical angle of the system 601; sucha configuration avoids appreciable effects due to total internalreflectance between the platen 617 and the skin site 119.

The presence of the platen 617 does not adversely interfere with theability to reduce the directly reflected light by use of polarizers. Thedetection subsystem 623 may include a polarizer 611 having an opticalaxis substantially orthogonal or parallel to the polarizer 607 comprisedby the illumination subsystem 621. Surface reflections at the interfacebetween the platen 617 and the skin site 119 are reduced in the casewhere polarizers 611 and 607 are oriented substantially orthogonal toeach other since light from the sample must undergo sufficiently manyscattering events to change its state of polarization before it can besensed by the detector 615. The detection subsystem 623 may additionallyincorporate detection optics that form an image of the region near theplaten surface 617 onto the detector 615. In one embodiment, thedetection optics 613 comprise a scanning mechanism (not shown) to relayportions of the platen region onto the detector 615 in sequence. Aninfrared filter 614 may be included to reduce the amount of infraredlight detected, particularly in embodiments where the detector 615 issensitive to infrared light, such as when a Bayer filter array is used.Conversely, as described above, the infrared filter 614 may be omittedin some embodiments and an additional light source 603 with emissions inthe infrared may be included in some embodiments.

As in the other arrangements described above, the detection subsystem623 is generally configured to be sensitive to light that has penetratedthe surface of the skin and undergone optical scattering within the skinand/or underlying tissue. The polarizers may sometimes be used to createor accentuate the surface features. For instance, if the illuminationlight is polarized in a direction parallel (“P”) with the platen 617,and the detection subsystem 623 incorporates a polarizer 611 in aperpendicular orientation (“S”), then the reflected light is blocked byas much as the extinction ratio of the polarizer pair. However, lightthat crosses into the skin site at a ridge point is optically scattered,which effectively randomizes the polarization (though the skin does havesome characteristic polarization qualities of its own, as is known tothose of skill in the art). This allows a portion, on the order of 50%,of the absorbed and re-emitted light to be observed by the S-polarizedimaging system.

A side view of one of the embodiments of the invention is shown with theschematic drawing provided in FIG. 7A. For clarity, this view does notshow the detection subsystem, but does show an illumination subsystem621 explicitly. The illumination subsystem 621 in this embodiment has aplurality of white-light sources 703 that are distributed spatially. Asshown in the drawing, the illumination optics 621 are configured toprovide flood illumination, but in alternative embodiments could bearranged to provide line, point, or other patterned illumination byincorporation of cylindrical optics, focusing optics, or other opticalcomponents as known to those knowledgeable in the art.

The array of white-light sources 703 in FIG. 7A need not actually beplanar as shown in the drawing. For example, in other embodiments,optical fibers, fiber bundles, or fiber optical faceplates or taperscould convey the light from the light sources at some convenientlocations to an illumination plane, where light is reimaged onto theskin site 119. The light sources could be controlled turning the drivecurrents on and off as LEDs might be. Alternatively, if an incandescentsource is used, switching of the light may be accomplished using someform of spatial light modulator such as a liquid crystal modulator orusing microelectromechanical-systems (“MEMS”) technology to controlapertures, mirrors, or other such optical elements. Such configurationsmay allow the structure of the sensor to be simplified. One embodimentis illustrated in FIG. 7B, which shows the use of optical fibers andelectronic scanning of illumination sources such as LEDs. Individualfibers 716 a connect each of the LEDs located at an illumination array710 to an imaging surface, and other fibers 716 b relay the reflectedlight back to the imaging device 712, which may comprise a photodiodearray, CMOS array, or CCD array. The set of fibers 716 a and 716 b thusdefines an optical fiber bundle 714 used in relaying light.

Another embodiment of a contact biometric sensor is shown schematicallywith the front view of FIG. 8. In this embodiment, the biometric sensor801 comprises one or more white-light illumination subsystems 823 and adetection subsystem 825. The illumination subsystems 823 comprise awhite-light source 803 that provides light that passes throughillumination optics 805 and a polarizer 807 to be directed to a platen817 over which a skin site is disposed 119. A portion of the light isdiffusely reflected from the skin site 119 into the detection subsystem825, which comprises imaging optics 815 and 819, a crossed polarizer811, and a dispersive optical element 813. The first imaging optics 819collimate light reflected from the skin site 119 for transmissionthrough the crossed polarizer 811 and dispersive element 813. Separatedspectral components are separately focused onto the detector 817 by thesecond imaging optics 815.

Contact biometric sensors like those illustrated in FIGS. 6-8 are alsoamenable to configurations in which the illumination region is inrelative motion with the skin site. As previously noted, such relativemotion may be implemented with a mechanism for scanning the illuminationlight and/or by moving the skin site. The presence of a platen incontact-sensor embodiments generally facilitates motion of the skin siteby confining a surface of the skin site to a defined plane; inembodiments where freedom of motion is permitted in three dimensions,additional difficulties may result from movement of the skin siteoutside the imaging depth. A swipe sensor may accordingly be implementedwith contact biometric sensors in a fashion as generally described inconnection with FIG. 4 above, but with a platen that prevents motion ofthe skin site in one direction. While in some embodiments, the swipesensor may be a stationary system, a contact configuration allows aroller system to be implemented in which the skin site is rolled over aroller structure that is transparent to the white light. An encoder mayrecord position information and aid in stitching a full two-dimensionalimage from a resulting series of image slices, as understood by those ofskill in the art. Light received from discrete portions of the skin siteis used to build up the image.

While the above descriptions of noncontact and contact biometric sensorshave focused on embodiments in which white light is used, otherembodiments may make use of other spectral combinations of light insimilar structural arrangements. In addition, other embodiments mayinclude additional variations in optical conditions to providemultispectral conditions. Some description of such multispectralapplications is provided in commonly assigned U.S. patent applicationSer. No. 10/818,698, entitled “MULTISPECTRAL BIOMETRIC SENSOR,” filedApr. 5, 2004 by Robert K. Rowe et al.; U.S. Pat. No. 11/177,817,entitled “LIVENESS SENSOR,” filed Jul. 8, 2005 by Robert K. Rowe; U.S.Prov. Pat. No. 60/576,364, entitled “MULTISPECTRAL FINGER RECOGNITION,”filed Jun. 1, 2004 by Robert K. Rowe and Stephen P. Corcoran; U.S. Prov.Pat. Appl. No. 60/600,867, entitled “MULTISPECTRAL IMAGING BIOMETRIC,”filed Aug. 11, 2004 by Robert K. Rowe; U.S. Pat. No. 11/115,100,entitled “MULTISPECTRAL IMAGING BIOMETRICS,” filed Apr. 25, 2005 byRobert K. Rowe; U.S. patent application Ser. No. 11/115,101, entitled“MULTISPECTRAL BIOMETRIC IMAGING,” filed Apr. 25, 2005; U.S. Pat. No.11/115,075, entitled “MULTISPECTRAL LIVENESS DETERMINATION,” filed Apr.25, 2005; U.S. Prov. Pat. Appl. No. 60/659,024, entitled “MULTISPECTRALIMAGING OF THE FINGER FOR BIOMETRICS,” filed Mar. 4, 2005 by Robert K.Rowe et al.; U.S. Prov. Pat. Appl. No. 60/675,776, entitled“MULTISPECTRAL BIOMETRIC SENSORS,” filed Apr. 27, 2005 by Robert K.Rowe; and U.S. patent application Ser. No. 11/379,945, entitled“MULTISPECTRAL BIOMETRIC SENSORS,” filed Apr. 24, 2006 by Robert K.Rowe. The entire disclosure of each of the foregoing applications isincorporated herein by reference for all purposes.

The noncontact and contact biometric sensors described above usewhite-light imaging in certain embodiments. The use of white lightpermits images to be collected simultaneously at multiple colors, withthe overall speed of data collection being faster than in embodimentswhere discrete states are collected separately. This reduceddata-collection time leads to a reduction in motion artifacts as theskin site moves during data collection. The overall sensor size may alsobe reduced and provided at lower cost by using a smaller number of lightsources when compared with the use of discrete illumination sources fordifferent colors. Corresponding reductions are also possible in theelectronics used to support coordinated operation of the light sources.In addition, color imagers are currently available at prices that aretypically lower than monochrome imagers.

The use of white-light imaging also permits a reduction in data volumewhen the sensor is designed to use all pixels in achieving the desiredresolution. For instance, a typical design criterion may provide a1-inch field with a 500 dots-per-inch resolution. This can be achievedwith a monochrome camera having 500×500 pixels. It can also be achievedwith a 1000×1000 color camera when extracting each color planeseparately. The same resolution can be achieved by using a 500×500 colorimager and converting to {R, G, B} triplets and then extracting themonochrome portion of the image. This is a specific example of a moregeneral procedure in which a color imager is used by converting toprimary-color triplets, followed by extraction of a monochrome portionof an image. Such a procedure generally permits a desired resolution tobe achieved more efficiently than with other extraction techniques.

4. Texture Biometric Sensor

Another form of contact biometric sensor provided in embodiments of theinvention is a texture biometric sensor. “Image texture” refersgenerally to any of a large number of metrics that describe some aspectof a spatial distribution of tonal characteristics of an image, some ofwhich were described above. For example, some textures, such as thosecommonly found in fingerprint patterns or wood grain, are flowlike andmay be well described by metrics such as an orientation and coherence.For textures that have a spatial regularity (at least locally), certaincharacteristics of the Fourier transform and the associated powerspectrum are important such as energy compactness, dominant frequenciesand orientations, etc. Certain statistical moments such as mean,variance, skew, and kurtosis may be used to describe texture. Momentinvariants may be used, which are combinations of various moments thatare invariant to changes in scale, rotation, and other perturbations.Gray-tone spatial dependence matrices may be generated and analyzed todescribe image texture. The entropy over an image region may becalculated as a measure of image texture. Various types of wavelettransforms may be used to describe aspects of the image texture.Steerable pyramids, Gabor filters, and other mechanisms of usingspatially bounded basis functions may be used to describe the imagetexture. These and other such measures of texture known to one familiarin the art may be used individually or in combination in embodiments ofthe invention.

Image texture may thus be manifested by variations in pixel intensitiesacross an image, which may be used in embodiments of the invention toperform biometric functions. In some embodiments, additional informationmay be extracted when such textural analysis is performed for differentspectral images extracted from a multispectral data set, producing achromatic textural description of the skin site. These embodimentsadvantageously enable biometric functions to be performed by capturing aportion of an image of a skin site. The texture characteristics of theskin site are expected to be approximately consistent over the skinsite, permitting biometric functions to be performed with measurementsmade at different portions of the image site. In many instances, it isnot even required that the portions of the skin site used in differentmeasurements overlap with each other.

This ability to use different portions of the skin site providesconsiderable flexibility in the structural designs that may be used.This is, in part, a consequence of the fact that biometric matching maybe performed statistically instead of requiring a match to adeterministic spatial pattern. The sensor may be configured in a compactmanner because it need not acquire an image over a specified spatialarea. The ability to provide a small sensor also permits the sensor tobe made more economically than sensors that need to collect completespatial information to perform a biometric function. In differentembodiments, biometric functions may be performed with purely spectralinformation, while in other embodiments, spatio-spectral information isused.

One example of a structure for a texture biometric sensor is shownschematically in FIG. 9A. The sensor 900 comprises a plurality of lightsources 904 and an imager 908. In some embodiments, the light sources904 comprise white-light sources, although in other embodiments, thelight sources comprise quasimonochromatic sources. Similarly, the imager908 may comprise a monochromatic or color imager, one example of whichis an imager having a Bayer pattern. The sensor 900 is referred toherein as a “contact” sensor because the image is collectedsubstantially in the plane of the skin site 119 being measured. It ispossible, however, to have different configurations for operating thesensor, some with the imager 908 substantially in contact with the skinsite 119 and some with the imager 908 displaced from the plane of theskin site 119.

This is shown for two illustrative embodiments in FIGS. 9B and 9C. Inthe embodiment of FIG. 9B, the imager 908 is substantially in contactwith the skin site 119. Light from the sources 904 propagates beneaththe tissue of the skin site 119, permitting light scattered from theskin site 119 and in the underlying tissue to be detected by the imager908. An alternative embodiment in which the imager 908 is displaced fromthe skin site 119 is shown schematically in FIG. 9C. In this drawing thesensor 900′ includes an optical arrangement 912 that translates an imageat the plane of the skin site 119 to the imager could comprise aplurality of optical fibers, which translate individual pixels of animage by total internal reflection along the fiber without substantiallyloss of intensity. In this way, the light pattern detected by the imager908 is substantially the same as the light pattern formed at the planeof the skin site 119. The sensor 900′ may thus operate in substantiallythe same fashion as the sensor 900 shown in FIG. 9B. That is, light fromthe sources 904 is propagated to the skin site, where it is reflectedand scattered by underlying tissue after penetrating the skin site 119.Because information is merely translated substantially without loss, theimage formed by the imager 908 in such an embodiment is substantiallyidentical to the image that would be formed with an arrangement likethat in FIG. 9A.

In embodiments where purely spectral information is used to perform abiometric function, spectral characteristics in the received data areidentified and compared with an enrollment database of spectra. Theresultant tissue spectrum of a particular individual includes uniquespectral features and combinations of spectral features that can be usedto identify individuals once a device has been trained to extract therelevant spectral features. Extraction of relevant spectral features maybe performed with a number of different techniques, includingdiscriminant analysis techniques. While not readily apparent in visualanalysis of a spectral output, such analytical techniques can repeatablyextract unique features that can be discriminated to perform a biometricfunction. Examples of specific techniques are disclosed in commonlyassigned U.S. Pat. No. 6,560,352, entitled “APPARATUS AND METHOD OFBIOMETRIC IDENTIFICATION AND VERIFICATION OF INDIVIDUALS USING OPTICALSPECTROSCOPY”; U.S. Pat. No. 6,816,605, entitled “METHODS AND SYSTEMSFOR BIOMETRIC IDENTIFICATION OF INDIVIDUALS USING LINEAR OPTICALSPECTROSCOPY”; U.S. Pat. No. 6,628,809, entitled “APPARATUS AND METHODFOR IDENTIFICATION OF INDIVIDUALS BY NEAR-INFRARED SPECTROSCOPY”; U.S.patent application Ser. No. 10/660,884, entitled “APPARATUS AND METHODFOR IDENTIFICATION OF INDIVIDUAL BY NEAR-INFRARED SPECTROSCOPY,” filedSep. 12, 2003 by Robert K. Rowe et al.; and U.S. patent application Ser.No. 09/874,740, entitled “APPARATUS AND METHOD OF BIOMETRICDETERMINATION USING SPECIALIZED OPTICAL SPECTROSCOPY SYSTEM,” filed Jun.5, 2001 by Robert K. Rowe et al. The entire disclosure of each of theforegoing patents and patent applications is incorporated herein byreference in its entirety.

The ability to perform biometric functions with image-textureinformation, including biometric identifications, may exploit the factthat a significant portion of the signal from a living body is due tocapillary blood. For example, when the skin site 119 comprises a finger,a known physiological characteristic is that the capillaries in thefinger follow the pattern of the external fingerprint ridge structure.Therefore, the contrast of the fingerprint features relative to theillumination wavelength is related to the spectral features of blood. Inparticular, the contrast of images taken with wavelengths longer thanabout 580 nm are significantly reduced relative to those images takenwith wavelengths less than about 580 nm. Fingerprint patterns generatedwith nonblood pigments and other optical effects such as Fresnelreflectance have a different spectral contrast.

Light scattered from a skin site 119 may be subjected to variety ofdifferent types of comparative texture analyses in differentembodiments. Some embodiments make use of a form of moving-windowanalysis of image data derived from the collected light to generate afigure of merit, and thereby evaluate the measure of texture or figureof merit. In some embodiments, the moving window operation may bereplaced with a block-by-block or tiled analysis. In some embodiments, asingle region of the image or the whole image may be analyzed at onetime.

In one embodiment, fast-Fourier transforms are performed on one or moreregions of the image data. An in-band contrast figure of merit C isgenerated in such embodiments as the ratio of the average or DC power toin-band power. Specifically, for an index i that corresponds to one of aplurality of wavelengths comprised by the white light, the contrastfigure of merit is

$C_{i} \equiv {\frac{{{\sum\limits_{\xi}{\sum\limits_{\eta}{{F_{i}( {\xi,\eta} )}}^{2}}}}_{R_{low}^{2} < {({\xi^{2} + \eta^{2}})} < R_{high}^{2}}}{{{F_{i}( {0,0} )}}^{2}}.}$

In this expression, F_(i)(ξ,η) is the Fourier transform of the imagef_(i)(x,y) at the wavelength corresponding to index i, where x and y arespatial coordinates for the image. The range defined by R_(low) andR_(high) represents a limit on spatial frequencies of interest forfingerprint features. For example, R_(low) may be approximately 1.5fringes/mm in one embodiment and R_(high) may be 3.0 fringes/mm. In analternative formulation, the contrast figure of merit may be defined asthe ratio of the integrated power in two different spatial frequencybands. The equation shown above is a specific case where one of thebands comprises only the DC spatial frequency.

In another embodiment, moving-window means and moving-window standarddeviations are calculated for the collected body of data and used togenerate the figure of merit. In this embodiment, for each wavelengthcorresponding to index i, the moving-window mean μ_(I) and themoving-window standard deviation σ_(I) are calculated from the collectedimage f_(i)(x, y). The moving windows for each calculation may be thesame size and may conveniently be chosen to span on the order of 2-3fingerprint ridges. Preferably, the window size is sufficiently large toremove the fingerprint features but sufficiently small to havebackground variations persist. The figure of merit C_(i) in thisembodiment is calculated as the ratio of the moving-window standarddeviation to the moving-window mean:

$C_{i} = {\frac{\sigma_{i}}{\mu_{i}}.}$

In still another embodiment, a similar process is performed but amoving-window range (i.e., max(image values)-min(image values)) is usedinstead of a moving-window standard deviation. Thus, similar to theprevious embodiment, a moving-window mean μ_(I) and a moving-windowrange δ_(I) are calculated from the collected image f_(i)(x,y) for eachwavelength corresponding to index i. The window size for calculation ofthe moving-window mean is again preferably large enough to remove thefingerprint features but small enough to maintain background variations.In some instances, the window size for calculation of the moving-windowmean is the same as for calculation of the moving-window range, asuitable value in one embodiment spanning on the order of 2-3fingerprint ridges. The figure of merit in this embodiment is calculatedas the ratio of the moving-window mean:

$C_{i} = {\frac{\delta_{i}}{\mu_{i}}.}$

This embodiment and the preceding one may be considered to be specificcases of a more general embodiment in which moving-window calculationsare performed on the collected data to calculate a moving-windowcentrality measure and a moving-window variability measure. The specificembodiments illustrate cases in which the centrality measure comprisesan unweighted mean, but may more generally comprise any other type ofstatistical centrality measure such as a weighted mean or median incertain embodiments.

Similarly, the specific embodiments illustrate cases in which thevariability measure comprises a standard deviation or a range, but maymore generally comprise any other type of statistical variabilitymeasure such as a median absolute deviation or standard error of themean in certain embodiments.

In another embodiment that does not use explicit moving-window analysis,a wavelet analysis may be performed on each of the spectral images. Insome embodiments, the wavelet analysis may be performed in a way thatthe resulting coefficients are approximately spatially invariant. Thismay be accomplished by performing an undecimated wavelet decomposition,applying a dual-tree complex wavelet method, or other methods of thesort. Gabor filters, steerable pyramids and other decompositions of thesort may also be applied to produce similar coefficients. Whatevermethod of decomposition is chosen, the result is a collection ofcoefficients that are proportional to the magnitude of the variationcorresponding to a particular basis function at a particular position onthe image. To perform spoof detection, the wavelet coefficients, or somederived summary thereof, may be compared to the coefficients expectedfor genuine samples. If the comparison shows that the results aresufficiently close, the sample is deemed authentic. Otherwise, thesample is determined to be a spoof. In a similar manner, thecoefficients may also be used for biometric verification by comparingthe currently measured set of coefficients to a previously recorded setfrom the reputedly same person.

5. Exemplary Applications

In various embodiments, a biometric sensor, whether it be a noncontact,contact, or texture sensor of any of the types described above, may beoperated by a computational system to implement biometric functionality.FIG. 10 broadly illustrates how individual system elements may beimplemented in a separated or more integrated manner. The computationaldevice 1000 is shown comprised of hardware elements that areelectrically coupled via bus 1026, which is also coupled with thebiometric sensor 1056. The hardware elements include a processor 1002,an input device 1004, an output device 1006, a storage device 1008, acomputer-readable storage media reader 1010 a, a communications system1014, a processing acceleration unit 1016 such as a DSP orspecial-purpose processor, and a memory 1018. The computer-readablestorage media reader 1010 a is further connected to a computer-readablestorage medium 1010 b, the combination comprehensively representingremote, local, fixed, and/or removable storage devices plus storagemedia for temporarily and/or more permanently containingcomputer-readable information. The communications system 1014 maycomprise a wired, wireless, modem, and/or other type of interfacingconnection and permits data to be exchanged with external devices.

The computational device 1000 also comprises software elements, shown asbeing currently located within working memory 1020, including anoperating system 1024 and other code 1022, such as a program designed toimplement methods of the invention. It will be apparent to those skilledin the art that substantial variations may be used in accordance withspecific requirements. For example, customized hardware might also beused and/or particular elements might be implemented in hardware,software (including portable software, such as applets), or both.Further, connection to other computing devices such as networkinput/output devices may be employed.

An overview of functionality that may be implemented with thecomputational device are summarized with the flow diagram of FIG. 11. Insome embodiments, a purported skin site is illuminated as indicated atblock 1104 with white light. This permits the biometric sensor toreceive light from the purported skin site at block 1108. As describedabove, the received light may be analyzed in a number of different waysin implementing a biometric function. The flow diagram shows how certaincombinations of analyses may be used in implementing the biometricfunction, although it is not necessary that all steps be performed. Inother instances, a subset of the steps may be performed, additionalsteps might be performed, and/or the indicated steps might be performedin a different order than indicated.

At block 1112, a liveness check may be performed with the received lightto confirm that the purported skin site is not some type of spoof,usually by verifying that it has the characteristics of living tissue.If a spoof is detected, an alert may be issued at block 1164. Thespecific type of alert that is issued may depend on the environment inwhich the biometric sensor is deployed, with audible or visual alertssometimes being issued near the sensor itself; in other instances,silent alerts may be transmitted to security or law-enforcementpersonnel.

The light received scattered from the purported skin site may be used atblock 1120 to derive a surface image of the purported skin site. Ininstances where the purported skin site is a volar surface of a finger,such a surface image will include a representation of the pattern ofridges and valleys on the finger, permitting it to be compared with adatabase of conventional fingerprints at block 1124. In addition oralternatively, the received light may be used to derive aspatio-spectral image at block 1128. This image may be compared with aspatio-spectral database having images that are associated withindividuals at block 1132. In either instance, the comparison may permitthe individual to be identified at block 1136 as a result of thecomparison. It is generally expected that higher-reliabilityidentifications may be made by using the full spatio-spectralinformation to provide a comparison with spatio-spectral images. But insome applications, there may be greater availability of conventionalfingerprint data, with some individuals having their fingerprints storedin large law-enforcement fingerprint databases but not inspatio-spectral databases. In such cases, embodiments of the inventionadvantageously permit the extraction of a conventional fingerprint imageto perform the identification.

The spatio-spectral data includes still additional information that mayprovide greater confidence in the identification, whether theidentification is made by comparison with a conventional fingerprintdatabase or through comparison with spatio-spectral information. Forexample, as indicated at block 1140, demographic and/or anthropometriccharacteristics may be estimated from the received light. When thedatabase entry matched to the image at block 1136 includes demographicor anthropometric information, a consistency check may be performed atblock 1144. For instance, an individual presenting himself may beidentified as a white male having an age of 20-35 years from theestimated demographic and anthropometric characteristics. If thedatabase entry against which the image is matched identifies theindividual as a 68-year-old black woman, there is a clear inconsistencythat would trigger the issuance of an alarm at block 1164.

Other information may also be determined from the received light, suchas an analyte concentration at block 1156. Different actions maysometimes be taken in accordance with the determined analyte level. Forexample, ignition of an automobile might be prohibited if ablood-alcohol level exceeds some threshold, or an alarm might be issuedif a blood-glucose level of a medical patient exceeds some threshold.Other physiological parameters, such as skin dryness conditions and thelike, may be estimated in other applications, with still other actionssometimes being taken in response.

FIG. 12 provides a similar flow diagram to illustrate applications of atexture biometric sensor. The sensor is used by positioning a skin siteof an individual in contact with the detector at block 1204. Aspreviously noted, the detector may be relatively small so that only aportion of a finger surface is positioned in contact with the detector;because of the nature of texture biometrics, variations in the specificportion of the surface placed in contact during different measurementsare not detrimental. Data are collected by illuminating the skin site atblock 1208 and receiving light scattered from the skin site with thedetector at block 1212.

The flow diagram indicates that different types of analyses may beperformed. It is not necessary that each of these analyses be performedin every case and, indeed, it is generally expected that in mostapplications only a single type of analysis will be used. One categoryof analysis, indicated generally at block 1216, uses purely spectralcomparisons of information. Another category of analysis, indicatedgenerally at blocks 1220 and 1228 uses image texture information bydetermining the image texture from spatio-spectral information in thereceived light at block 1220 and comparing that image texture with adatabase of texture biometric information at block 1228. With either orboth types of analysis, a biometric function is performed, such asidentification of the individual at block 1232.

Thus, having described several embodiments, it will be recognized bythose of skill in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the invention. Accordingly, the above description should notbe taken as limiting the scope of the invention, which is defined in thefollowing claims.

1-27. (canceled)
 28. A noncontact biometric system comprising: anillumination source configured to illuminate a purported skin site of anindividual with light having a first polarization condition, wherein thepurported skin site is positioned to interact with the polarized lightwhile not being in contact with any solid surface; a detection subsystemconfigured to collect light scattered from the purported skin sitehaving a second polarization condition; and a computational unitinterfaced with the detection subsystem configured to: derive aplurality of spatially distributed images of the purported skin sitefrom the received light, and analyze the plurality of spatiallydistributed images to perform a biometric function.
 29. The noncontactbiometric system according to claim 28, wherein the first polarizationcondition and the second polarization condition are orthogonal.
 30. Thenoncontact biometric system according to claim 28, wherein the firstpolarization condition and the second polarization condition areparallel.
 31. The noncontact biometric system according to claim 28,wherein the illumination source comprises a polarizer configured topolarize the light according to the first polarization condition. 32.The noncontact biometric system according to claim 28, wherein thedetection subsystem comprises a polarizer configured to polarize thelight according to the second polarization condition.
 33. The noncontactbiometric system according to claim 28, wherein the detection subsystemis configured to configured to collect light scattered from thepurported skin site having a third polarization condition, wherein thesecond polarization condition and the third polarization are orthogonal.34. The noncontact biometric system according to claim 33, wherein thesecond polarization condition and the first polarization are orthogonal.35. The noncontact biometric system according to claim 33, wherein thethird polarization condition and the first polarization are parallel.36. A noncontact biometric system comprising: an illumination sourceconfigured to illuminate a purported skin site of an individual; a firstpolarizer configured to polarize the light from the illumination sourcewith a first polarization condition prior to illuminating the purportedskin site; a second polarizer configured to polarize the light scatteredfrom the purported skin with a second polarization condition, whereinthe second polarization condition is cross-polarized with the firstpolarization condition; a third polarizer configured to polarize thelight scattered from the purported skin with a third polarizationcondition, wherein the third polarization condition isparallel-polarized with the first polarization condition; a detectionsubsystem configured to collect the polarized light from the purportedskin site through the second polarizer and the third polarizer; and acomputational unit interfaced with the detection subsystem configured toderive a plurality of spatially distributed images of the purported skinsite from the received light, and analyze the plurality of spatiallydistributed images to perform a biometric function.
 37. The noncontactbiometric system according to claim 36, wherein the purported skin siteis positioned to interact with the polarized light while not being incontact with any solid surface.
 38. A noncontact biometric detectionmethod comprising: illuminating a purported skin site located within atarget space with light having a first polarization condition, whereinthe purported skin site is positioned to interact with the polarizedlight while not being in contact with any solid surface; receivingpolarized light scattered from the purported skin site under a secondpolarization condition; deriving a plurality of spatially distributedimages of the purported skin site from the received polarized light; andanalyzing the plurality of spatially distributed images to perform thebiometric function.
 39. The noncontact biometric detection methodaccording to claim 38, wherein the plurality of spatially distributedimages corresponding to different volumes of illuminated tissue of theindividual.
 40. A noncontact biometric detection method comprising:illuminating a purported skin site located within a target space withlight having a first polarization condition; polarizing a portion of thelight scattered from the purported skin site under a second polarizationcondition; polarizing a portion of the light scattered from thepurported skin site under a third polarization condition, wherein thesecond polarization condition and the third polarization condition arecross-polarized; deriving a plurality of spatially distributed images ofthe purported skin site from the polarized light; and analyzing theplurality of spatially distributed images to perform the biometricfunction.
 41. The noncontact biometric detection method according toclaim 40, wherein the second polarization condition and the firstpolarization condition are cross-polarized.
 42. The noncontact biometricdetection method according to claim 40, wherein the third polarizationcondition and the first polarization condition are parallel-polarized.43. The noncontact biometric detection method according to claim 40,wherein the purported skin site is positioned to interact with thepolarized light while not being in contact with any solid surface. 44.The noncontact biometric detection method according to claim 40, whereinthe plurality of spatially distributed images corresponding to differentvolumes of illuminated tissue of the individual.
 45. A noncontactbiometric sensor comprising: a illumination subsystem disposed toilluminate a purported skin site of an individual, wherein the purportedskin site is positioned to interact with the polarized light while notbeing in contact with any solid surface; a detection subsystem disposedto receive light scattered from the purported skin site; and acomputational unit interfaced with the detection subsystem and having:instructions for deriving a plurality of spatially distributed images ofthe purported skin site from the received light, the plurality ofspatially distributed images corresponding to different volumes ofilluminated tissue of the individual; and instructions for analyzing theplurality of spatially distributed images to perform a biometricfunction.
 46. The noncontact biometric sensor according to claim 45,wherein the illumination subsystem is configured to illuminate thepurported skin site with white light.
 47. The noncontact biometricsensor according to claim 45, wherein detection subsystem comprises acolor imager on which the received light is incident.
 48. The noncontactbiometric sensor according to claim 45, wherein the illuminationsubsystem is configured to illuminate the purported skin site withpolarized light having a first polarization condition; and wherein thedetection subsystem comprises a polarizer configured to filter thereceived light according to a second polarization condition.
 49. Thenoncontact biometric sensor according to claim 48, wherein the firstpolarization condition and the second polarization condition arecross-polarized relative to one another.
 50. The noncontact biometricsensor according to claim 48, wherein the first polarization conditionand the second polarization condition are parallel-polarized relative toone another.
 51. A method of performing a noncontact biometric function,the method comprising: illuminating a purported skin site of anindividual, wherein the purported skin site is positioned to interactwith the polarized light while not being in contact with any solidsurface; receiving light scattered from the purported skin site;deriving a plurality of spatially distributed images of the purportedskin site from the received light with the color image; and analyzingthe plurality of spatially distributed images to perform the biometricfunction.
 52. The method according to claim 51, wherein the illuminatingthe purported skin site of an individual comprises illuminating the skinsite of the individual with white light.
 53. The method according toclaim 51, wherein the plurality of spatially distributed imagescorrespond to different volumes of illuminated tissue of the individual.